Surgery for obstructive hypertrophic cardiomyopathy (septal myotomy/myectomy or mitral valve replacement) improves symptoms in many, but not all patients with hypertrophic cardiomyopathy. To assess which hemodynamic changes before and following surgery best correlate with exercise benefit following surgery, 14 patients with hypertrophic cardiomyopathy underwent treadmill exercise testing (Bruce protocol) with measurement of maximum oxygen consumption before and 6 months following surgery. The post-operative exercise study demonstrated improved exercise duration but with only a marginal improvement in maximum oxygen consumption. The improvement in maximum oxygen consumption following surgery compared to the preoperative study correlated directly with the magnitude of reduction in left ventricular outflow gradient and left ventricular end-diastolic pressure. The 5 patients with no improvement in maximum oxygen consumption, including 2 with greater than 50 mmHg gradient reduction, had less than 5 mmhg reduction in left ventricular end-diastolic pressure. Seven of the 9 patients with increased post-operative maximum oxygen consumption had a gradient reduction greater than or equal to 65 mmhg; the 2 with lesser gradient reduction had greater than 8 mmhg reduction in left ventricular end-diastolic pressure. Thus, benefit in exercise capacity following surgery may be determined as much by the reduction in the left ventricular filling pressures as by the magnitude of gradient reduction following surgery.